Cobalt oxide catalysts for wet lean methane combustion

• Author / Creator

  Somaye Nasr

• Over the past several decades, there have been environmental concerns over anthropogenic emissions of methane, highlighting the need to minimize methane emissions. Natural gas has been employed as one of the alternatives for gasoline combustion engines. Methane is the main constituent of natural gas. Due to the high stability of methane molecules, its complete oxidation is not possible in a combustion engine. One way to minimize methane emissions from combustion engine exhaust would be using a catalytic converter. Increasing interest in natural gas (NG) fueled vehicle has increased the demand for research on a suitable catalyst. Flameless catalytic methane combustion has been investigated extensively as a means of reducing pollutant emissions and enhancing combustion efficiency. The main benefit of this method is the relatively low reaction temperature and low production of NOx. This technology enables industry to employ a safer and cleaner approach for energy production. It is well established that palladium and platinum are efficient methane combustion catalysts. Unfortunately, the high price of these precious metals restricts their practical applications. The goal of our research is to identify an optimal catalyst that partially substitutes noble metals with mixed transitional oxides, while minimally reducing the catalyst activity and significantly reducing the material’s cost. To achieve this goal, the research focused on using non-PGM transition metals as a promoter and as a support for Pd catalysts used for lean methane combustion reaction under wet conditions. Cobalt was chosen because of its low oxygen bonding energy and its relatively high activity in oxygen and methane activation among 3d elements. At first, reaction kinetics of methane combustion was investigated on Co3O4 and Pd/Co3O4 (0.27 wt.% Pd) catalysts for a fuel-lean feed. Significant cobalt oxide contribution to the activity of the bimetallic catalyst was observed, especially at higher water concentrations and lower temperatures (up to 70 %). The Co3O4 contribution was not only in performing the methane combustion itself but also in supplying surface oxygen rather than in affecting the activation energy. The kinetic evidence shows that the observed behaviour of Pd/Co3O4 catalyst is the effect of strong metal-support interactions (SMSI).
  To shed light on the chemical states of Pd and Co in bimetallic catalysts an in-situ X-ray absorption spectroscopy (XAS) study can also be a viable investigation. The Co3O4 and Pd/Co3O4 catalysts were analyzed for Pd and Co speciation during wet lean methane combustion at temperature ranges below 450 °C by means of in situ X-ray absorption spectroscopy. The contributions from metallic Pd, PdO and Pd (OH)2 and metallic Co, Co (II), and Co(III) oxides were quantified as a function of temperature. It is concluded that Pd/CoOx system the formation of methane combustion inactive Pd (OH)2 seems to be suppressed as compared to the Al2O3 system. The results revealed that Pd activates and supplies oxygen to Co-O suprafacial active sites, and therefore, enhances the Co-catalyzed methane combustion performance. As the cobalt oxide surface is more resistant to water poisoning than PdO, the reaction order with respect to water is affected in a positive way, as Co sites contribute to methane combustion because of Pd feeding activated oxygen to them. It is concluded that CoOx does not affect the degree of hydroxylation of the Pd surface.
  After Pd and Co speciation, the next goal was to design a practical catalyst that is relatively cheap while exhibiting high activity under wet condition. To do so it was hypothesized that the use of ceria oxide in the cobalt oxide system would enhance the low dispersion of cobalt, hence improving the catalyst activity. Ceria was chosen due to its high oxygen storage capacity and wide use in converter formulations. The Co3O4/CeO2 catalyst demonstrated the same performance after 100 hours on stream, suggesting that the ceria stabilized the Co3O4 nanoparticles against sintering. In addition, due to the large Co3O4 crystals the metal-support interactions are improbable. Cobalt deposition did not impact the activation energy comparing to the bulk catalyst but did increase the catalyst active sites.
  Overall, the cobalt and the interaction with Pd in the Pd/Co3O4 catalyst was studied and the results showed enhancement in the catalyst activity by supplying oxygen to the Co-O active sites. Presence of water vapor deactivates the Co3O4/CeO2 catalyst, but the results revealed the reversible impact. Collectively, this study deepens our knowledge of cobalt role in catalyst system employed for lean methane combustion specifically under wet environment.

• Subjects / Keywords

  • Methane Lean Combustion
  • X-ray absorption spectroscopy
  • Catalytic Methane Oxidation
  • Cobalt Oxide

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-pb84-da90

• License

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